1. Field of the Invention
The invention relates to power amplifier valves, such as, for example, tetrodes.
2. Discussion of Background
The higher the power that valves are capable of delivering, the more energy losses in the valve have to be taken into account and eliminated in order not to risk bringing about deterioration or destruction of the valve by abnormal heating.
In valves operating at high frequency, losses are produced due especially to the circulation of currents at high frequency in the grids located between the cathode and the anode of the valves. In the case of a tetrode especially, the screen grid, most often called G2 grid, is traversed by high-frequency currents circulating vertically between the top and the bottom of the grid. These currents arise because the grid is placed in a resonant output circuit of the valve, and because, in every circuit resonating at high frequency, regions of standing waves are set up with nodes and antinodes of currents and of voltage. The highest ultra-high-frequency currents are certainly produced at current antinodes.
The grid is then subjected to very significant heating. It is not known precisely how to measure this heating (inside a closed vacuum valve) but the appearance of reverse grid currents has been noted when operating at very high power and high frequency. In other words, whereas the normal grid current is a consumption of current in one direction, it is noted that an increase in the operating power of the valve leads to the reversal of the direction of passage of the current in the grid connection. It has been noted, for example, that the grid current passed very rapidly from a normal positive value of a few amperes to a negative value of a few amperes (in a few seconds) during start-up of a tetrode.
This reversal of the grid current leads to the supposition that the grid starts to emit electrons in great quantity (whereas it ought not to do so). This emission of electrons is probably brought about by the increase in temperature of the grid. In fact, the material employed for the grid is most often of pyrolytic graphite, which has a relatively poor emissivity at the normal temperature of operation of the valve. It is thus probable that it is a very significant abnormal heating of the grid which confers a high thermo-emissivity on it. The quantity of current which can be measured, suggests that the grid attains temperatures of the order of 2000.degree. C. Only such temperatures can in fact explain the appearance of such a high reverse grid current.
This high temperature of the grid may be the cause of malfunctions of the valve: the grid radiates a very significant quantity of heat towards the colder parts of the valve and brings about an abnormal outgassing of the latter. The ions liberated in the valve are then sources of electrical breakdowns, instances of tripping-off, etc. The grid insulation ceramics can deteriorate (cooling cracks) under the action of heat. In any event the result is a reduction in the reliability and the lifetime of the valves.
The aim of the invention is to reduce the risks of malfunction which seem to be due to an abnormal increase in temperature of the grid, in the valves whose grid is placed in a circuit resonating at high frequency and is traversed by high-frequency currents set up by this resonance.